Solidify modifier creates edges with a different thicknesses
Here is an example:



The figure on the top here has "even thickness" unchecked.

That means the modifier makes so all segments length are the same. But as you can see the flat parts are uneven because the corner has the same length as the extremities.

The figure on the bottom has "even thickness" checked.

Here the flat parts are regular. But the segment at the corner does not have the same size compared to the extremities.

Now look at this figure:



When "even thickness" is checked, Blender calculates the intersection of 2 flat parts. Like intersecting 2 rectangles. That means (as shown with the green segment here above) that the length at the corner must be larger than the others.